In the semiconductor processing industry, photoresist is normally applied to semiconductor wafers by means of a spin coating machine. The wafer is placed on a flat vacuum chuck and spun slowly while the photoresist is applied through a nozzle near the center of the wafer. When the wafer is spun at high speed, centrifugal force causes the photoresist to flow towards the circumference of the wafer, thereby covering the surface of the wafer with a smooth, even coat of photoresist.
This process creates two disposal problems. First, an excess of photoresist flows off the edge of the wafer and must be collected and disposed of. Second, the photoresist contains a solvent which vaporizes as the process takes place. These vapors normally flow into the factory exhaust system and must be removed before the exhaust is released to the atmosphere.
FIG. 1 illustrates the photoresist drainage system which has been in use by the industry for a number of years. A coating machine 100 is illustrated schematically as containing a spin motor 101 connected to a flat vacuum chuck 102 upon which a wafer 103 is placed. Spin motor 101 is positioned below a catch basin 104.
A liquid drain line 105, an air purge line 106 and a safety line 107 lead from coating machine 100. As shown in FIG. 2, which is a cross-sectional view of coating machine 100, lines 105 and 106 connect to a coating chamber 100A and line 107 connects to a motor chamber 100B of coating machine 100. Lines 105 and 106 have inlets which are positioned near the spinning wafer so as to purge the vapor in the coat chamber with air and collect the excess liquid photoresist. Safety line 107 has an inlet 108 which is located near spin motor 101. Line 107 does not collect liquid photoresist but functions as a safety line for removing solvent vapors from motor chamber 100B. These vapors are highly volatile and could ignite if they reach excessive concentrations around spin motor
Also shown in FIG. 2 are a cup rinse ring 104A and a splash guard 104B. Cup rinse ring 104A contains a number of small holes through which solvent is sprayed to rinse the interior walls of coating chamber 100A Cup rinse ring 104A also contains an edge bead removal nozzle 104C which removes the photoresist built up on the edge of the wafer during coating, as described in U.S. Pat. No. 4,518,678. Cup rinse ring 104A and edge bead removal nozzle 104C are major sources of the solvent which must be disposed of.
Referring to FIG. 1, lines 105 and 106 lead to a drain tank 109, which is used to collect the liquid photoresist. When the liquid photoresist in drain tank 109 reaches a certain level, drain tank 109 is removed and the liquid photoresist is emptied into the factory chemical reclaim system, or the tank can be drained in place through an attached drain pipe.
Line 107 leads to an exhaust manifold 110, which is connected to the factory exhaust system. A connecting line 111 connects drain tank 109 directly to exhaust manifold 110, so that vapors evaporating from the liquid photoresist in drain tank 109 can be removed via exhaust manifold 110 and the factory exhaust system. Exhaust manifold 110 slopes slightly towards a drain line 110A which connects with the factory drainage system. Any vapors which condense in exhaust manifold 110 run into drain line 110A.
When the system is operating, the factory exhaust system provides a suction on exhaust manifold 110 which tends to pull vapors from motor chamber 100B through line 107 and from drain tank 109 by means of connecting line 111. Since lines 105 and 106 are connected to drain tank 109, this suction also tends to pull vapors and air from coating chamber 100A. As each wafer is processed, in order to prevent the photoresist from drying too quickly, a selected concentration of solvent vapor is maintained in the atmosphere in coating chamber 100A. To accomplish this, as illustrated in FIG. 3, a butterfly valve 111A is positioned in conduit 111, under the control of a programmable exhaust unit 112. During the spin coating process, if the programmable exhaust is used, programmable exhaust unit 112 closes the butterfly valve 111A so as to restrict the flow of air and vapor from drain tank 109 to exhaust manifold 110. This in turn limits the flow of vapor and air in lines 105 and 106 and maintains the vapor concentration in coating chamber 100A at an increased level. When the coating process is completed, programmable exhaust unit 112 opens butterfly valve 111A and the normal flow of the solvent vapors and air through lines 105 and 106 into the factory exhaust system is reestablished. For safety reasons, the flow from motor chamber 100B through line 107 is not totally restricted.
The arrangement illustrated in FIG. 1 introduces a considerable amount of solvent vapor into the factory exhaust system. A large proportion of this vapor results from evaporation at the surface of the liquid photoresist and solvents in drain tank 109. Between coating cycles, when butterfly valve 111A is open, a gaseous vapor mixture is sucked through lines 105 and 106. This gaseous mixture flows into drain tank 109 where it creates turbulent flow patterns which increase the rate of solvent evaporation at the liquid surface. This evaporation rate increases as the liquid level in tank 109 gets higher.
With the increasingly stringent air pollution requirements, a substantial portion of the solvent vapors must be removed from the factory exhaust system before they reach the atmosphere. Reducing the amount of vapors that enter the exhaust system in the first place would help solve this problem and reduce the costs of environmental compliance. This invention accomplishes both of these purposes.